In a conventional time signal-synchronized analog clock, sometimes referred to as an “atomic clock,” a clock movement is kept in synchronization with a standard time signal broadcast by radio from a remote transmitting station. In the clock, the hand positions are determined by a hand position detector, usually an optical system in which the 12:00:00 position of the hour, minute and second hands is determined by detecting the alignment of three holes in the drive gearing with a fixed fourth hole, one hole corresponding to each hand. The hand positions are compared electronically with time information, referred to as a “time stamp,” derived from the broadcast time signal. A motor driver that operates two clock motors, one operating the second hand, and the other operating the minute and hour hands, periodically effects an appropriate retarding or advancing movement of the hands in response to the comparison between the hand position information and the time stamp.
The broadcast time signal usually used is a so-called “DCF” signal having a carrier frequency of 77.5 kHz. The amplitude of the signal is reduced by 25% at the beginning of each second except for the 59th second. The duration of the reduction in amplitude is utilized to convey information. An amplitude reduction over an interval of 200 milliseconds (ms) can be interpreted as a logical “high” and an amplitude reduction over an interval of 100 ms can be interpreted as a logical “low”. Thus, over an interval of one minute, sixty bits of information can be transmitted. The information transmitted each minute as binary data can include the calendar year, month, day, hour, and minute as well as time zone information, time changes from daylight to standard or from standard to daylight, astronomical time corrections (“leap seconds”), parity checks, and other miscellaneous information. FIG. 1 is a schematic diagram of a typical conventional time signal-synchronized clock. The clock comprises a clock movement 20, a motor driver 22, a radio receiver 24, a clock hand position detector 26, and a processor 28 for decoding the received radio signal to produce a time stamp, comparing the time stamp to the detected clock hand position, and sending an adjustment command to the motor driver 22. The unit is powered by a 1.5 volt “AA” electrochemical cell 30.
FIG. 2 illustrates the combined amplitude and pulse-width modulation of the carrier, and the logical bit information derived therefrom. FIG. 3 illustrates the portion of the encoded information corresponding to minutes and hours.
The conventional time-signal-synchronized clock operates satisfactorily in locations where the broadcast time signal is sufficiently strong. The conventional system, however, also depends on the reception of radio signals transmitted over hundreds or even thousands of miles. Because these signals are weak at the point of reception, satisfactory reception is highly dependent on position, antenna orientation, terrain, the proximity of shielding materials such as metal objects, and weather conditions, and even time of day. In some localities remote from the transmitting antenna, and in underground locations and other locations where the receiver is shielded, the received signal may be too weak to provide reliable time synchronization.
In addition, in the case of conventional time signal-synchronized clocks, it is difficult to set each one of a set of clocks to a different time zone, and difficult to set different clocks to receive time signals from different sources. Some conventional “atomic” clocks in the United States include a time zone switch enabling the user to select, Eastern, Central, Mountain, or Pacific time, but cannot easily be set to display time in other time zones.